FPAs are typically comprised of a two-dimensional array of Infrared (IR) radiation detectors. The individual IR detector elements may be organized in a regular row and column, mosaic-type fashion Such an array of IR detector elements can be comprised of, for example, HgCdTe, InSb, Multiquantum Well Superlattice (MQW SL) material or doped silicon semiconductor material. The IR detector induced signal from each of the IR detector elements is typically coupled to an electronic interface circuit such as a CTIA, a source follower direct readout or a charge coupled device, where each of the signals are integrated over an interval of time and subsequently read out by a suitable multiplexing circuit. Electronic circuits with HgCdTe detector arrays for processing IR induced signals require a significant degree of material compatibility to achieve a long lifetime and reliable operation. Also, interface electronic circuits require low device and circuit noise characteristics for obtaining a satisfactory signal-to-noise ratio. For some applications the circuits must tolerate nuclear radiation in high nuclear radiation environments and also consume low power to achieve both weight and size reduction.
Typically, readout chips are coupled to, or "bumped", with detector arrays at room temperature using indium bump technology. When the chips are rapidly cooled from room temperature to a cryogenic operating temperature stress can build up between the readout circuit and the detector array through the bumping. This stress is most severe if the coefficients of thermal expansion between the materials are different and/or if the chips are large in area. Silicon, the conventional semiconductor material employed for readout circuits, has a poor thermal expansion match with detector materials such as HgCdTe, InSb and GaAs based MQWL SL IR detectors However, other materials such as GaAs are known to have a coefficient of thermal expansion that is closer to that of these detector materials than is the coefficient of expansion of silicon.
It is therefore one object of the invention to provide a GaAs FPA signal processor that exhibits a low power consumption and an improved immunity to nuclear radiation (radiation hardness).
It is a further object of the invention to provide a GaAs FPA signal processor that exhibits low noise when employed in a FPA signal processor
It is still one further object of the invention to provide a GaAs FPA signal processor array that has a coefficient of thermal expansion which is closely matched to that of the material of an associated IR detecting array.
Another object of the invention is to provide a GaAs FPA signal processor array that includes a differential pair having an improved load transistor circuit that is substantially insensitive to processing-related variables.